This invention generally relates to an apparatus and method for the controlled evaporation of liquids, including solutions. More particularly, the preferred embodiment of this invention comprises an improved apparatus and method for evaporating the solvent of a solution on a drop-by-drop basis. A preferred embodiment of the invention also provides means for measuring the amount of the solute remaining after the solvent has substantially evaporated.
It is known that the amount of solute contained in a quantity of solution may be measured by a variety of means after the solvent of the solution has evaporated. Current methods for the deposition and measurement of solvent non-volatile residue (NVR), however, suffer from a variety of problems.
The most common method involves removing the solvent and thereby concentrating the residue by atmospheric evaporation of 100 to 1000 mL of liquid solvent followed by weighing the residue. This method has many inherent errors and problems that may result in substantial errors in measurement, particularly at concentration levels (solute/solvent) of less than 3 mg/L or its equivalent. One additional problem associated with boiloff evaporation and release of the vapor to the atmosphere is that the process may be hazardous to the environment and to laboratory workers. Even when the solvent is partially removed by in vacuo distillation, the final 20-25 mL are routinely released to the environment by thermal evaporation. A variety of ASTM methods specify the evaporative removal of solvents followed by weighing of residues, such as ASTM F331, E1235M, and E1731M.
Other more recent methods of evaporating solvents and/or of measuring non-volatile residue in solvents are described in U.S. Pat. Nos. 5,679,580; 5,560,889; 5,374,396; and 5,698,774.
It would be desirable if the evaporation of a liquid, such as a solution, could be controlled on a drop-by-drop basis. It would be further desirable if the measurement of solvent non-volatile residue could be performed accurately, particularly at concentrations of less than 3 mg/L or its equivalent. Finally, it would be desirable if the controlled evaporation of a solution and the accurate measurement of the NVR did not result in any significant release of hazardous material.
Among the advantages of the invention is the controlled evaporation of a liquid, such as a solution, on a drop-by-drop basis. Another advantage of the invention is the accurate measurement of NVR, particularly at concentrations of less than 3 mg/L. A further advantage of the invention is the reduction in the amount of hazardous material released into the environment as a result of the evaporation of the liquid or solvent.
Additional objects and advantages of this invention will become apparent from an examination of the drawings and the ensuing description.
As used herein, the term solvent non-volatile residue or NVR refers to any substance contained in a solution, which substance is substantially not susceptible to evaporation at common temperatures and pressures and which substance remains after evaporation of the solvent of the solution.
As used herein, the term inert gas refers to a gas or mixture of gases which are generally unreactive and lacking active chemical or physiological properties, which may include, but are not limited to the inert gases of Group 0 of the periodic table of elements as well as other similar gases, elements, compounds and mixtures that exist in the gaseous state at common atmospheric pressures and temperatures. Nitrogen is the preferred inert gas for use in connection with this invention, but others may be used.
As used herein, the term droplet refers to a small amount of liquid or solution that falls separately from a depositing device, such as a syringe, and adheres to a deposition surface plate in a generally round-shaped globule.
As used herein, the term ultra-clean refers to a condition exceeding that of common laboratory conditions in terms of the elimination of foreign matter and contamination from the environment.
The present invention provides an automatic, controlled method and apparatus for the evaporation of a volume of a liquid, such as a solution (including a volume of less than about 5 mL). Where the liquid is a solution containing non-volatile residue (xe2x80x9cNVRxe2x80x9d), the invention provides for deposition of NVR into a small controlled area on a suitable clean deposition surface plate for subsequent measurement by one of several available methods. Preferably, the deposition surface plate is essentially flat or concave.
More particularly, this invention provides a feedback-controlled automated method for the deposition of a volume of liquid, such as a solution, using a controlled motor or other suitable device to intermittently push down or actuate a depositing device, such as a syringe plunger, in order to deposit a successive series of small droplets onto a deposition surface plate. The actuation of the depositing device is controlled in response to a determination that at least a portion of the liquid in a droplet on the deposition surface plate has evaporated. In other words, the evaporative disappearance of each successive droplet is monitored so that the next droplet is deposited only when the preceding droplet has at least partially evaporated. This process automatically continues until the desired amount of liquid has substantially evaporated and, in the case where the liquid is a solution containing NVR, the non-volatile residue has been accumulated on the deposition surface plate.
A deposition needle with a very small internal diameter (e.g. less than about 0.5 mm) and preferably having a flat or blunt open end is employed as part of the depositing device so that capillary forces act to prevent inadvertent deposition of droplets. The temperature of the deposition surface may be measured and controlled to increase the evaporation rate of higher boiling point liquids or solvents. A flow of a purge gas, generally gaseous nitrogen or another inert gas, is preferably employed to sweep across the deposition surface plate in a circular pattern (or other pattern generally flowing around or across the deposition surface plate), to aid in the proper evaporative conditions, to prevent any possible vapor/oxygen explosive conditions, and to maintain the deposition surface plate and the deposited residue in an environmentally clean condition.
A variety of liquid or solvent droplet detection methods may be used to trigger the deposition process so that the desired volume will be deposited in sequential droplets automatically. One preferred method is the appearance of increased light to a suitable light sensitive detector as the result of the evaporation of at least a portion of the droplet. Another such method involves the change in spectral distribution of reflected light as at least a portion of the liquid or solvent in the droplet evaporates, which change is detected photo-electrically. Yet another method involves the measurement of the decreasing concentration of already evaporated liquid or solvent vapor in the purge gas as at least a portion of the liquid or solvent evaporates. Still another method involves monitoring the change in the weight on the deposition surface plate caused by the evaporation of at least a portion of the liquid or solvent that is deposited thereon. Another method of merit involves the use of piezo-electric sensing of the evaporation of at least a portion of each successive droplet.
Yet another method is the detection of light from a suitable light source using a video camera that is focused on a portion of the deposited droplet in such a way that the presence of the droplet obscures the light from the source. This physical obscuring of the light from the source disappears as at least a portion of the droplet evaporates. The wetting of the liquid or solvent to the needle and the deposition surface plate can be used to advantage in controlling in part the physical shape of the droplet.
Following completion of the successive depositions, the amount of NVR remaining can readily be determined quantitatively by a variety of methods down to and below about 1 microgram per milliliter (which is equivalent to about 1 mg per liter or, for a solvent density of 1, about one part per million or PPM). Thus, for example, the method detailed in U.S. Pat. No. 3,297,874 and subsequent continuations-in-part (all now expired) permits the quantitative measurement of residue in amounts as low as 1 nanogram on an ultra-clean deposition surface.
Another method for measuring residues quantitatively and qualitatively in amounts less than about 1 microgram involves the use of very low grazing angle FTIR technology. Yet another method for determining the presence of residues in such amounts is through use of a quartz crystal micro-balance in which a clean quartz crystal is used as the deposition surface. Other methods known to those having ordinary skill in the art to which the invention relates may also be used. In all cases, calibration of the overall method may be carried out simply by evaporating solvents with known amounts of residues and determining the appropriate relationships of the amount of deposited residues to the measurement responses.
The invention may be employed to evaporate solvents such as those primarily of an organic nature, such as hexanes and other hydrocarbons, halogenated hydrocarbons, CFC-113 and replacements for CFC-113, other halogenated solvents, alcohols, ketones, esters, organic acids, and the like, although aqueous solvents may also be evaporated according to the invention. In addition, liquids and solvents boiling below about 130xc2x0 Celsius may be evaporated according to the invention at 1 atmosphere, whereas higher boiling point liquids and solvents may be evaporated according to the invention in vacuo.